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Changes in plant growth, photosynthetic gas exchange, chlorophyll fluorescence and stem diameter of soybean [Glycine max (L.) Merr.] plants under drought stress were studied. Total plant dry mass was reduced by 30 % compared to well-watered control plants. Leaf water potential was slightly decreased by water stress. Water stress induced daytime shrinkage and reduced night-time expansion of stem. Photosynthetic rate, stomatal conductance and transpiration rate were significantly declined by water stress, while the intercellular CO2 concentration was changed only slightly at the initiation of stress treatment. The maximum photochemical efficiency of photosystem 2 and apparent photosynthetic electron transport rate were not changed by water stress.

The ability to synthesize and accumulate glycine betaine is wide-spread among angiosperms and is thought to contribute to salt and drought tolerance. In plants glycine betaine is synthesized by the two-step oxidation of choline via the intermediate betaine aldehyde, catalyzed by choline monooxygenase and betaine aldehyde dehydrogenase (BADH). Two sorghum (Sorghum bicolor) cDNA clones, BADH1 and BADH15, putatively encoding betaine aldehyde dehydrogenase were isolated and characterized. BADH1 is a truncated cDNA of 1391 bp. BADH15 is a full-length cDNA clone, 1812 bp in length, predicted to encode a protein of 53.6 kD. The predicted amino acid sequences of BADH1 and BADH15 share significant homology with other plant BADHs. The effects of water deficit on BADH mRNA expression, leaf water relations, and glycine betaine accumulation were investigated in leaves of preflowering sorghum plants. BADH1 and BADH15 mRNA were both induced by water deficit and their expression coincided with the observed glycine betaine accumulation. During the course of 17 d, the leaf water potential in stressed sorghum plants reached -2.3 MPa. In response to water deficit, glycine betaine levels increased 26-fold and proline levels increased 108-fold. In severely stressed plants, proline accounted for >60% of the total free amino acid pool. Accumulation of these compatible solutes significantly contributed to osmotic potential and allowed a maximal osmotic adjustment of 0.405 MPa.

Pairs of homozygous near-isogenic glycinebetaine-containing (Bet1/Bet1) and -deficient (bet1/bet1) F8 lines of Zea mays L. (maize) were tested for differences in salt (150 mM NaCl or 127.25 mM NaCl plus 22.5 mM CaCl2) tolerance. The Bet1/Bet1 lines exhibited less shoot growth inhibition (as measured by dry matter accumulation, leaf area expansion rate and/or, plant height extension rate) under salinized conditions in comparison to their nearisogenic bet1/bet1 sister lines. These growth differences were associated with maintenance of a significantly higher leaf relative water content, a higher rate of carbon assimilation, and a greater turgor in Bet1/Bet1 lines than in bet1/bet1 lines under salinized conditions. These results strongly suggest that a single gene conferring glycinebetaine accumulation (and/or a tightly linked locus) plays a key role in osmotic adjustment in maize.

A series of near-isogenic glycinebetaine-containing and -deficient F8 pairs of Zea mays L. (maize) lines were developed. The pairs of lines differ for alternative alleles of a single locus; the wild-type allele conferring glycinebetaine accumulation is designated Bet1 and the mutant (recessive) allele is designated bet1. The near-isogenic lines were used to investigate whether glycinebetaine deficiency affects the pool size of the glycinebetaine precursor, choline, using a new method for glycinebetaine and choline determination: stable isotope dilution plasma desorption mass spectrometry. Glycinebetaine deficiency in maize was associated with a significant expansion of the free choline pool, but the difference in choline pool size was not equal to the difference in glycinebetaine pool size, suggesting that choline must down-regulate its own synthesis. Consistent with this, glycinebetaine deficiency was also associated with the accumulation of the choline precursor, serine. A randomly amplified polymorphic DNA marker was identified that detects the bet1 allele. In 62 F8 families tested the 10-mer primer 5[prime]-GTCCTCGTAG produced a 1.2-kb polymerase chain reaction product only when DNA from Bet1/bet1 or bet1/bet1 lines was used as template. All 26 homozygous Bet1/Bet1 F8 families tested were null for this marker.

Field experiments were conducted with maize to determine the contribution of husk leaves, laminae extending from the husk, to grain production and their characteristics of mineral nutrition and photosynthesis in comparison to culm leaves. The measurement of leaf area, photorespiration rate and 13C natural abundance of the husk leaves, and grain yield in flint corn hybrid differing in husk leaf size were made after silking. In a separate experiment, the effect of N application rate on leaf area and apparent photosynthetic rate (Po) in the husk leaves was also examined. Husk leaves alone produced 45 to 79% of grain yield relative to the control plants. In spite of different genotypes, a close correlation between the husk leaf area and grain yield was observed. Po and 13C natural abundance of the husk leaves were almost equivalent to those of culm leaves, indicating that the photosynthesis of husk leaf belongs to C4 type. At the lower N rate (50 kg ha-1), husk leaf area decreased, but Po remained stable, indicating that under suboptimal conditions of N nutrition, leaf development in the husk leaves is more severely impaired than Po. These results suggest that the husk leaves have a higher efficiency in grain production than culm leaves.

An experiment was conducted to examine the effect of pod removal during the pod filling period on nitrate reduction in soybean (Glycine max [L.] cv. Tamahomare) grown under field and hydroponic conditions. Pod removal led to a significant decrease in the leaf nitrate reductase activity 10–20 d after treatment (DAT). 15N tracer experiment revealed that the accumulation rate of reduced-N in a detached shoot decreased by pod removal. These results indicate that the leaf nitrate reductase activity is regulated by the sink activity of pods for N at the grain filling stage. Pod removal increased the nitrate-N content and did not affect significantly the carbohydrate content in the leaves, indicating that nitrate and energy supply might not be involved in the inhibition of nitrate reductase activity.

A field experiment was conducted to study the factors that control leaf area development in husk leaves of flint corn especially the amount of sugars and N in cell wall. The ear leaf of Fi flint corn (N-19 × X-15), was subjected to 13CO2 feeding 8 days before silking (DBS) under field conditions. The leaf area and the total sugars of the various cell components of the husk leaves were measured 13 DBS. The leaf area of the husk leaves at 9 DBS was about 8% of that at the silking stage. The total sugars of methanol-soluble fraction (cytosol) accounted for about 68% of the whole husk leaf, while hemicellulose and cellulose fractions constituted less than 10% at 9 DBS. At the silking stage, the total sugars of hemicellulose and cellulose fractions increased by 23% and 56% respectively. In the husk leaves, the percentage of plant 13C was about 6.6%, and 13C incorporated in the cellulose fraction increased by 74% at 24h after the 13CO2 feeding. These results suggest that the development of the husk leaf area is closely related to the accumulation of cell wall materials such as hemicellulose and cellulose.